Add lesson notices filters

[renatho]

Part of #5068

Changes proposed in this Pull Request

• It adds some filters allowing to customize links used in lesson notices (login, registration and take course).
  • It was implemented as filters only because our Learning Mode block is a generic block for any type of notices.

Testing instructions

• Add the following filters to your site:

  add_filter(
  	'sensei_lesson_take_course_url',
  	function( $take_course_url, $course_id ) {
  		return 'http://lesson-take-course.com?' . $course_id;
  	},
  	10,
  	2
  );
  
  add_filter(
  	'sensei_registration_url',
  	function( $registration_url, $redirect ) {
  		return 'http://registration.com?' . $redirect;
  	},
  	10,
  	2
  );
  
  add_filter(
  	'sensei_login_url',
  	function( $registration_url, $redirect ) {
  		return 'http://login.com?' . $redirect;
  	},
  	10,
  	2
  );

• Create a course with lessons, and Learning Mode enabled.
  • In WP admin > Settings > General, enable the setting "Anyone can register".
  • Logged-out, access a lesson from the created course.
  • Make sure the "Take course" button was filtered to "lesson take" URL, and "Sign in" to "registration" URL.
  • In WP admin > Settings > General, disable the setting "Anyone can register".
  • Make sure not the "Sign in" link is filtered to "login" URL, and take course continues with the "lesson take" URL.
  • Log-in, access the lesson, and make sure that clicking on "Take course" will automatically enroll the user.
• Create a course with lessons, and Learning Mode disabled.
  • Access a lesson from the created course logged-out and logged-in. Make sure in both situations you see the notice linking to the "lesson take" URL.

New/Updated Hooks

• sensei_lesson_take_course_url - Filter the take course URL displayed in lessons. Notice that in Learning Mode, when user is logged-in, it will not use this filter and directly enroll the user in the course.
• sensei_registration_url - Filter the registration URL.
• sensei_login_url - Filter the login URL.

[jom]

Looks good and works well!